libview: Make caret cursor blink

Based on GtkEntry and GtkTextView implementation, the caret cursor
blinks when the view is active and caret navigation is enabled. It stops
blinking after a while if there's no user interaction. It uses
GtkSettings:gtk-cursor-blink-time and
GtkSettings:gtk-cursor-blink-timeout.

https://bugzilla.gnome.org/show_bug.cgi?id=702076